Tandem accelerators typically use a charging system and are used to produce beams of charged particles having a very high energy such as, for example, 10 MeV. These beams can then be used for various applications, such as, for example, basic nuclear physics studies, cancer therapy, radioisotope production, and materials analysis. Unfortunately, conventional tandem generators cannot be properly controlled by their voltage feedback systems when the terminal voltage (i.e., the voltage at a terminal or terminal cell inside the tandem accelerator) falls below a certain voltage level such as 5-10% of the maximum operating voltage. For example, the voltage cannot be regulated below approximately 0.3 MV for 3 MV conventional tandem generator, and for a 25 MV generator, voltage regulation is lost around approximately 1-2 MV.
Typically, a Tandem Van de Graff-type accelerator, or more simply, a Tandem has a column having a low-energy or first beam tube (or column) and a high-energy or second beam tube (or column), a stripper cell, beam steering means, a terminal cell (e.g., including an electrode) which is disposed between the high- and low-energy beam tubes, and a charging system, all of which are contained within a pressure vessel which provides proper pressure (e.g., either pressure or vacuum) for proper electrical isolation. The charging system typically comprises a moving belt or a chain comprising an insulating material which is used to convey a charge to the terminal via, for example, a “pickoff pulley,” as disclosed in, for example, U.S. Pat. No. 3,353,107, entitled “High Voltage Particle Accelerators Using Charge Transfer Processes,” to Van de Graaff (the '107 patent) which is incorporated herein by reference in its entirety. Van de Graaff discloses a power source comprising a single insulating belt which is located within a pressure vessel. As mentioned above, this system is difficult to regulate below a certain voltage value and is therefore not suitable for certain uses.
In Tandems, as well as other electrostatic accelerators, a generating Voltmeter (GVM) is typically used to measure the voltage on the terminal. The GVM comprises a rotating and stationary vane assembly in close proximity with each other, with the rotating vane assembly facing the terminal. The GVM generates a signal that is proportional to the voltage on the terminal. The terminal voltage is controlled by either the feedback from a corona discharge between the terminal and sharp needles, known as corona points, or feedback from the accelerated beam itself as it is deflected and its beam current recorded by slits. This feedback is then used to control the terminal voltage.
Although there have been attempts to use other power sources in tandem accelerators, such as disclosed in U.S. Pat. Nos. Re 34,575 (the '575 patent) and 6,414,327, entitled “Electrostatic Ion Accelerator” and “Method and Apparatus for Ion Beam Generation,” respectively, both to Klinkowstein, which are incorporated herein by reference in their entirety. Similarly to the '107 patent, these two patents disclose power supplies located within a pressure vessel used to house the beam tubes. Further, the solid state rectifier-type power supply disclosed by the '575 patent is subject to “sag” which is attributable to AC impedance of capacitors in earlier stages and a voltage ripple effect which is amplified by the number of stages used. Accordingly, it is difficult to control the voltage at the terminal of the '575 patent.
Another type of tandem accelerator is disclosed in U.S. Pat. No. 5,293,134, entitled “Tandem Accelerator,” to Holmes, which is incorporated herein by reference in its entirety, and which discloses using vacuum rather than an insulating gas within a pressure vessel. Further, Holmes discloses integrating accelerating and electric stress shields which are used as capacitors and are different from the beam tube which is taught by the present invention.
Further, it is difficult to maintain a corona discharge current, which is necessary to control the terminal voltage, in the absence of a beam, at terminal voltages which are lower than a threshold value in conventional tandems.
Accordingly, there is a need for an apparatus capable of providing stable low-voltage operation for a tandem accelerator which can be accurately and easily controlled when the terminal voltage is below the aforementioned threshold level.
Further, there is a need for a tandem accelerator which can provide a moderately low-energy beam having an energy which is suitable for high-resolution analytical techniques such as, for example, Nuclear Reaction Analysis (NRA).